1. Field of the Invention
The present invention relates to a noise removal apparatus equipped in car radios etc. for removing noise such as multipath noise and pulse noise from received radio signals, and an FM receiver provided with the same.
2. Background Art
Conventionally, radio receivers are equipped with a noise removal apparatus for removing noise from received radio signals. For example, in car radios mounted on automobiles, as the received radio signals sometimes include multipath noise, these car radios are provided with an apparatus for removing these multipath noise.
Multipath noise is caused by the fact that carrier radio signals are reflected by reflecting objects such as mountains or high-rise buildings. To describe the same more concretely, this multipath noise is generated by the fact that when a direct wave transmitted from a radio station and a reflected wave reflected by a reflecting object are interfered each other and received by a car radio, due to phase difference between the direct wave and the reflected wave, a part of the direct wave is eliminated by the reflected wave. In such a case, the quality of the reproduced voice by the car radio is reduced.
Multipath noise is, as shown in FIG. 15; upper diagram, represented by the envelopes when viewed macroscopically. Whereas, when it is viewed microscopically, as shown in FIG. 15; lower diagram, it is understood that they are aggregates of drastically changing spike-like noise. Therefore, multipath noise have a higher frequency relative to the radio signals.
Consequently, in conventional car radios, multipath noise is removed by removing high frequency elements from radio signals with a LPF (Low Pass Filter) etc. Also, in car radios that are able to make stereo demodulation, multipath noise is removed by removing high frequency stereo elements from received radio signals with a LPF etc. to left monaural elements only.
However, even when these technologies are applied, it was difficult to remove multipath noise sufficiently. That is to say, depending on various situations, sometimes a large high frequency multipath noise is generated exceeding an assumption value. In such a case, even when the high frequency elements are simply removed or converted into monaural, uncleared noise is still remained.
Now, different from such a way that multipath noise is removed to the full by simply cutting the frequency with an LPF etc., it is conceivable to detect noise themselves and remove the relevant noise. To describe the same more in detail, according to the proposed technique, high frequency elements are extracted from received radio signals with an HPF (High Pass Filter) etc. As multipath noise is of higher frequency relative to radio signals, it is possible to detect multipath noise with this processing. After that, correction processing is made on detected multipath noise for a period of generation duration of each spike-like noise. That is to say, for a period of generation duration of the spike-like noise, values immediately before the same or compensation values are output. By virtue of this manner, it is made possible to remove multipath noise.
By the way, radio signals include not only multipath noise but also pulse noise. Pulse noise is represented by spark plug noise and motored mirror noise. Same as multipath noise, frequency of the same is higher relative to radio signals by nature. Therefore, when high frequency elements are extracted from radio signals with an HPF as the manner of proposed technique, not only multipath noise but pulse noise is also detected. Accordingly, in the proposed technique, both multipath noise and pulse noise is removed without being differentiated from each other.
Pulse noise have, ordinarily, longer generation duration relative to multipath noise by nature. Accordingly, even when this pulse noise is removed, as its generation density is low, there is no particular problem. However, depending on various situations, there may be a case when pulse noise is generated frequently. In such case, there exists such a problem that large correction errors are generated frequently and output voice signals are largely distorted resulting in deterioration of the voice quality.
FIG. 16A is a diagram illustrating radio signals including multipath noise; FIG. 16B is a diagram illustrating radio signals including pulse noise having relatively long generation frequency; FIG. 16C is a diagram illustrating radio signals including multipath noise having relatively short generation frequency.
Generation duration Δt of each spike-like noise constituting multipath noise is, as shown in FIG. 16A, relatively short. On the contrary, generation duration Δt of pulse noise is, as shown in FIG. 16B and FIG. 16C, relatively long. Accordingly, when pulse noise is removed, comparing to a case where multipath noise is removed, the distortions of radio signals become greater than the distortion thereof.
To describe the same more in detail, when spike-like noise is removed, a portion of FM demodulated signals from which the noise is removed becomes as shown with a broken line in FIG. 16. In this case, the correction error dr, which is a difference between an original value and a corrected value, is, as shown in FIG. 16A, relatively small. On the contrary, when pulse noise is removed, as shown in FIG. 16B, the correction error dr becomes relatively large. Accordingly, when pulse noise is removed, comparing to a case where multipath noise is removed, a large influence is rendered to the deterioration of the voice quality.
However, as shown in FIG. 16B, in case where the generation frequency is relatively long, the deterioration of quality still remains within allowable range. Whereas, in case where the generation frequency is relatively short, large correction errors dr occur frequently and a large distortion is rendered to the radio signals resulting in a drastic deterioration of the voice quality.